Hepatitis B is a serious disease that attacks the liver and can remain silent without symptoms for years.

In a surprise announcement by the U.S. Surgeon General, the U.S. Public Health Service and the American Academy of Pediatrics, the federal policy is now described as deeming the inoculation proper for use at birth only if the baby's mother tests positive for the disease or the mother's status is unknown.

If the mother tests negative, Surgeon General David Satcher said, federal health agencies now counsel giving the much-criticized shot later in infancy -- between two and six months of age -- "when the infant is considerably larger" and the baby's immune system is further developed.

Satcher said premature babies should not receive the vaccine until reaching full-term gestational age (9 months, 2 weeks, and a weight of at least 5.5 pounds). Federal health officials did not acknowledge that recent criticism from Congress, parents and doctors played any role in the policy change.

Instead, recommendations for the shot were promulgated last week almost as a footnote in a larger announcement on halting the use of thimerosal, a type of mercury added in traces to many vaccines as a preservative and bacteria killer. Mercury is a highly toxic metallic element that can build up in the body if ingested frequently enough.

Satcher stressed there is no evidence children have been harmed by their exposure to thimerosal in the current immunization schedule that includes an ever-increasing number of shots -- as many as 19 in some states -- that babies under 18 months of age must receive.

But the surgeon general said "because any potential risk is of concern" the Public Health Service, Academy of Pediatrics and vaccine makers "agree that thimerosal-containing vaccines should be removed as soon as possible." The hepatitis B vaccine contains thimerosal.

Satcher said there is a "margin of safety" in all vaccines using thimerosal and "to increase that margin even further" pediatricians and parents can now "postpone the first dose of hepatitis B vaccine from birth until two to six months of age" when the mother tests disease-free.

The Department of Health and Human Services and other federal health agencies have been under pressure by the Food and Drug Administration since December to "assess the risk of all mercury-containing food and drugs" to conform to the FDA Modernization Act of 1997, authored by Rep. Dan Burton, R-Ind., chairman of the House Government Reform Committee.

Critics of the shots for newborns -- required in many states and recommended by the federal Centers for Disease Control since 1991 -- point out that not everyone is exposed to the disease.

People deemed to be at high risk for hepatitis B are those engaging in unprotected sex with infected partners, drug addicts who use unsterile needles, health workers who handle blood and police and medical emergency workers.